The electron and the plum pudding

Until the late 1800s, everyone thought atoms could not be broken. Then scientists found that some elements give off invisible energy and particles all by themselves. This is radioactivity. If particles come out of atoms, then atoms must contain smaller particles.

The experiment. In 1897, J. J. Thomson passed electricity through a gas at very low pressure in a glass tube. A glowing beam, called cathode rays, travelled from the cathode (–) to the anode (+).

Reasoning, step by step:

• In electric and magnetic fields, the rays were pulled towards the + side. Opposites attract, so the rays must be made of negative particles.
• These particles were much lighter than atoms. So the atom must have smaller parts.
• The rays were the same whatever metal the cathode was made of and whatever gas was in the tube. So these particles are in every atom.

They were named electrons.

An electron's charge is tiny: –1.602 × 10⁻¹⁹ coulomb. To keep things simple, we call it –1.

"Atoms are neutral. So where is the positive charge?" Thomson's answer: the atom is a ball of positive charge with electrons spread through it, the plum pudding model. Picture a watermelon: the red pulp is the positive charge, and the seeds are the electrons. The positive and negative charges balance, so the atom is neutral.

Thomson won the Nobel Prize in Physics in 1906. He led the famous Cavendish Laboratory, where he inspired a young scientist named Ernest Rutherford. (The colours in pictures of atoms are only to help you see; atoms have no colours.)